Holography is a technique that enables three-dimensional (3-D) images to be generated, recorded, and later displayed. It may involve the use of a laser, interference, diffraction, light intensity and phase recording, and suitable illumination of the recording. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thereby making the image appear in 3-D. The holographic recording itself is not an image, since when viewed it appears to be made up of an apparently random structure of varying intensity, density, or profile. Holographic displays, also known as holographic video or electro-holography, compute the interference pattern directly, present that pattern on a spatial light modulator (SLM) and, together with suitable illumination, produce an updatable holographic image.
As the demand for 3-D displays rapidly grows, holographic displays are considered by many within the 3-D entertainment industry as the ultimate goal of 3-D displays. Such high regard is held for these devices because holographic displays are capable of displaying all the 3-D cues of a real scene to a viewer. These cues include stereopsis, vergence, accommodation, parallax, occlusion, and shading. Unfortunately, to date, designing and fabricating holographic displays have proven difficult due to one or more difficult challenges that have limited display size, field of view, frame rate, and/or prevented providing both horizontal and vertical parallax.
In this regard, to create a large holographic display with a wide field of view (fov), the pitch of the display's spatial light modulator (SLM) must be fine (e.g., less than 1 micrometer (μm) and more typically less than 0.5 μm for an approximately 30° fov) over the entire large area of the display (e.g., 127 millimeters (mm) by 100 mm or the like). Unfortunately, the pitches of most common SLMs, such as digital micromirror devices (DMDs) or liquid crystal on silicon (LCOS) devices, are typically only as coarse as about 5 to 10 μm and are the same horizontally and vertically, providing only 1° to 3° fov. Further, the pitches are only maintained over small areas in these devices such as over 20 mm by 20 mm. Demagnifying optics can be used to increase the pitch and field of view but at the generally unacceptable expense of the image size (and vice versa) due to the Lagrange Invariant (i.e., for an optical system of only lenses, the product of the image size and ray angle is constant).
In some attempts to provide an improved holographic display, multiple SLMs have been tiled together to increase either the size or field of view of the hologram. With simple spatial tiling of multiple SLMs to increase the size of the hologram, however, there are noticeable seams in the holographic image due to gaps between the SLMs from the borders and electronics. Spatial tiling a single SLM has also been achieved using replication optics or using 2-axis scanners. Gaps and misalignments in the spatial tiling appear at the hologram plane and visually interfere with and confuse the 3-D imagery. Multiple SLMs have also been arranged in an arc, with precision optical mounts, to increase the field of view. The holographic images overlap in the center of the arc, a far distance from the SLMs, with a corresponding reduction in the holographic image's resolution the further the distance from the SLM. Several of these systems use an asymmetric diffusing screen, producing horizontal parallax only (HPO) images. Some also use acousto-optical modulators (AOMs) capable of providing traveling acoustic waves of pitches of about 5 μm over larger lengths. These large lengths can be arranged into widths of about 1 meter by heights of about 100 mm. However, to cancel the motion of the traveling waves, descanning optics and scanners are required. Also, other optics may be required to create higher pitches at the expense of display width. Further, the acoustic waves only diffract in one direction, and the resulting hologram is necessarily HPO.
Due to the horizontal arrangement of the human eyes, horizontal parallax is more important than vertical parallax for binocular stereopsis and motion parallax. This fact is often used in horizontal parallax only (HPO) holographic displays to reduce computation and data bandwidth requirements compared to full parallax holographic displays. However, the appearance of the HPO hologram does not change with vertical motion of the viewer and their viewing location or point of view. In other words, a single viewer may move their head up and down vertically (e.g., be sitting or squatting and then stand up), and the hologram's appearance would not change as would a true 3-D object. In some artistic and entertainment applications, especially those provided for single stationary viewers, the loss of vertical parallax may be acceptable.
Vertical parallax is important to fix absolute position in space. In many 3-D display implementations, the loss of vertical parallax is not acceptable, which has led some experts in the 3-D display industry to argue that a holographic display that is HPO is a “non-starter.” For example, in implementations involving interaction with the hologram or involving multiple viewers that collaborate (e.g., point to or interact with the same location on the holographic image), the holographic display will be ineffective unless there is at least a small amount of vertical parallax. Such “limited vertical parallax” may be necessary for the viewers to see or experience a consistent scene from differing points of view. Due to human kinetics (e.g., it is easier for humans to shift their views left and right than up and down), the amount of desirable vertical parallax is often much lower than a desirable amount of horizontal parallax.
Hence, there is a need for holographic displays or holographic display systems that address some of these challenges. Preferably, such new holographic displays would provide a relatively large 3-D image or hologram and would provide some amount of vertical parallax (e.g., provide limited vertical parallax). An issue, though, facing such development is that providing different amounts of information and fields of view in the horizontal and vertical directions is difficult with current full parallax holographic displays. With common square pixel SLMs, the horizontal and vertical pitches and, therefore, the fields of view are the same (unless anamorphic optics are used, which often is not desirable due to astigmatic aberrations, cost, manufacturing and design complexity, and other concerns).
Further, there is a need to provide techniques and algorithms for generating the content to be displayed on such holographic displays. Similarly, there is a need to be able to increase the speed of that content generation as well as to be able to store the content in a reasonable amount of memory space.
It is against this background that the techniques described herein have been developed.